Torsional potentials and full-dimensional simulation of electronic absorption and fluorescence spectra of para-phenylene oligomers using the semiempirical self-consistent charge density-functional tight binding approach

Organic Photoredox Catalysts for CO2 reduction: Driving Discovery with Genetic Algorithms

This work implements a genetic algorithm (GA) to discover organic catalysts for photoredox CO2 reduction that are both highly active and resistant to degradation. The LUMO energy of the ground state catalyst is chosen as the activity descriptor and average Mulliken charge on all ring carbons as the descriptor for resistance to degradation via carboxylation (both obtained using density functional theory), to construct the fitness function of the GA. We combine the results of multiple GA runs, each based on different relative weighting of the two descriptors, and rigorously assess GA performance by calculating electron transfer barriers to CO2 reduction. A large majority of GA predictions exhibit improved performance relative to experimentally studied o-, m-, and p-terphenyl catalysts. Based on stringent cut-offs imposed on the average charge, barrier to electron transfer to CO2, and excitation energy, we recommend 25 catalysts for further experimental investigation of viability towards photoredox CO2 reduction.

Impact of molecular chain length on polarizabilities of model acenes and oligomers

A systematic quantum chemical study of model linear oligomers and their ring-fused (condensed) analogues based on six-membered and five-membered aromatic units is presented. Static electric polarizabilities were calculated for optimal geometries. The dependence of electronic and vibrational polarizability contributions on the molecular size was discussed and polymer limits were estimated. The presence of a diradical electronic structure in fused six-membered compounds significantly increases the polarizability values.

Computational Analysis of Electron Transfer Kinetics for CO2 Reduction with Organic Photoredox Catalysts

We present a fundamental description of the electron transfer (ET) step from substituted oligo(p-phenylene) (OPP) radical anions to CO₂, with the larger goal of assessing the viability of underexplored, organic photoredox routes for utilization of anthropogenic CO₂. This work varies the electrophilicity of para-substituents to OPP and probes the dependence of rate coefficients and interfragment interactions on the substituent Hammett parameter, σ_p, using constrained density functional theory (CDFT) and energy decomposition analysis (EDA). Large electronic couplings across substituents indicates an adiabatic electron transfer process for reactants at contact. As one might intuitively expect, free energy changes dominate trends in ET rate coefficients in most cases, and rates increase with substituent electron-donating ability. However, we observe an unexpected dip in rate coefficients for the most electron-donating groups, due to the combined impact of flattening free energies and a steep increase in reorganization energies. Our analysis shows that, with decreasing σ_p, flattening OPP LUMO levels lower the marginal increase in free energy. EDA reveals trends in electrostatics and charge transfer interactions between the catalyst and substrate fragments that influence free energy changes across substituents. Reorganization energies do not exhibit a direct dependence on σ_p and are largely similar across systems, with the exception of substituents containing lone pairs of electrons that exhibit significant deformation upon electron transfer. Our study therefore suggests that while a wide range of ET rates are observed, there is an upper limit to rate enhancements achievable by only tuning the substituent electrophilicity.

TORSIONAL POTENTIALS AND FULL-DIMENSIONAL SIMULATION OF ELECTRONIC ABSORPTION SPECTRA OF para-PHENYLENEVINYLENE OLIGOMERS USING SEMIEMPIRICAL HAMILTONIANS

A systematic study of torsional potential curves in electronic ground state based on second-order Møller–Plesset energy (MP2), density functional theory (DFT), and Austin mode 1 (AM1) methods is presented for para-phenylenevinylene oligomers constructed from two to four aromatic rings. The semiempirical AM1 approach gives the correct location of potential energy minima in comparison with the reference MP2 calculations and literature data. However, the semiempirical AM1 energy barriers at perpendicular orientation are ca. 30% smaller than the MP2 ones. The DFT calculations indicate optimal planar structures and the barriers are three times higher than MP2 values. Excited-state potential energy curves evaluated from vertical excitations at the time-dependent density functional theory (TD-DFT) and ab initio CI levels exhibit much steeper increase of values in the vicinity of perpendicular orientation than in the semiempirical Zerner's intermediate neglect of differential overlap (ZINDO) and ab initio RI-CC2 cases. The effects of vibrational motion of phenylene rings on the torsional broadening of absorption spectra were estimated from semi-classical molecular dynamics simulations and harmonic oscillator sampling. The simulated spectra agree well with the experiment and allow estimating the conformer distribution of the molecules.